The invention relates to a frequency converter.
The invention is based on the object of providing a frequency converter having a high efficiency.
The invention achieves this object by means of a frequency converter for generating at least one frequency converter output voltage which is used for driving an electric motor, wherein the at least one frequency converter output voltage has a prescribable frequency converter output voltage amplitude and a prescribable frequency converter output voltage frequency, the frequency converter comprising: a clocked DC/DC converter which is designed to generate from an input direct voltage comprising an input voltage level a DC/DC converter output voltage having a prescribable DC/DC converter output voltage level, wherein the DC/DC converter is designed to generate the DC/DC converter output voltage level in dependence on the prescribable frequency converter output voltage amplitude, and a clocked inverter comprising a number of controllable switches, to which inverter the DC/DC converter output voltage is applied and which is designed to actuate the switches with an inverter switching frequency in such a manner that the at least one frequency converter output voltage with the prescribable frequency converter output voltage frequency is generated from the DC/DC converter output voltage.
The frequency converter is designed to generate at least one drive voltage or frequency converter output voltage which is used for driving an electric motor. The at least one frequency converter output voltage is typically an AC voltage. The frequency converter output voltage can be a phase voltage of an electric motor. The at least one frequency converter output voltage has an adjustable frequency converter output voltage amplitude and an adjustable frequency converter output voltage frequency. The frequency converter output voltage frequency determines, for example, a frequency of rotation of a resultant magnetic field and thus a rotational speed of the electric motor, wherein the frequency converter output voltage amplitude determines, for example, the torque produced by means of the electric motor. In this respect, the relevant technical literature is also referred to.
The frequency converter has a clocked DC/DC converter which is designed to generate from an input direct voltage having an input voltage level a DC/DC converter output voltage having a DC/DC converter output voltage level, wherein input voltage level and DC/DC converter output voltage level can be different. The DC/DC converter is designed to generate the DC/DC converter output voltage level in dependence on the prescribable, prescribed or desired frequency converter output voltage amplitude. The DC/DC converter can generate the DC/DC converter output voltage level in such a manner that it corresponds to the prescribable frequency converter output voltage amplitude or is equal to the prescribable frequency converter output voltage amplitude, respectively.
The frequency converter also has a clocked inverter having a number of controllable switching devices which, for example, can be a component of a bridge circuit (for example a three-phase transistor bridge). The inverter has applied to it the DC/DC converter output voltage and is designed to actuate its switching devices independently of the prescribable frequency converter output voltage amplitude with a respective inverter switching frequency in such a manner that the at least one frequency converter output voltage with the prescribable frequency converter output voltage frequency is generated from the DC/DC converter output voltage.
The inverter switching frequency, i.e. the switching frequency of a respective switching means of the inverter can correspond to the frequency converter output voltage frequency, i.e. the inverter switching frequency and the frequency converter output voltage frequency can be identical, i.e. the inverter is operated with fundamental clocking or block clocking, respectively.
The DC/DC converter can be designed to generate the DC/DC converter output voltage level in dependence on the prescribable frequency converter output voltage amplitude and additionally in dependence on a precontrol signal or modulation signal. The frequency converter or a control unit of the frequency converter which, for example, controls the operation of the frequency converter and generates associated drive signals for all components to be activated, can also be designed to generate the precontrol or modulation signal and, in particular, to generate it in such a manner that a torque ripple is minimized.
The precontrol or modulation signal can be, for example, a sinusoidal or a rectified sinusoidal signal which has a frequency which is a multiple, for example a three- to six-fold of the frequency converter output voltage frequency. An amplitude of the precontrol or modulation signal can depend, for example, on the frequency converter output voltage amplitude and/or on the frequency converter output voltage frequency. The precontrol or modulation signal, particularly the amplitude and/or the frequency of the precontrol or modulation signal can also be generated in dependence on the following quantities:                motor voltages,        motor currents,        a nominal/actual angular position of a rotor of the electric motor activated,        a nominal/actual angular position of a voltage vector corresponding to the output voltages,        a nominal/actual angular position of a current vector and/or        any combinations of the quantities mentioned above.        
Naturally, the frequency converter can have sensors suitable for detecting the above-mentioned quantities.
For example, the DC/DC converter can be designed to generate the DC/DC converter output voltage level in dependence on the prescribable or prescribed frequency converter output voltage amplitude and additionally in dependence on the precontrol signal or modulation signal, in such a manner that the DC/DC converter output voltage level corresponds to a sum or to a difference of the prescribable or prescribed frequency converter output voltage amplitude and the precontrol or modulation signal.
By means of the precontrol or modulation signal, the increased torque ripple due to the fundamental clocking or block clocking at low rotational speeds can be reduced or eliminated, respectively, by precontrolling the DC chopper.
The frequency converter can be designed for generating precisely three frequency converter output voltages which then, for example, form the phase voltages of a three-phase electric motor in order to actuate, for example, a three-phase AC motor.
The controllable switches of the inverter and/or one or more controllable switches of the DC/DC converter can be unipolar circuit breakers, for example MOSFETs. At least two of the controllable switches of the inverter and/or of the DC/DC converter can be connected in parallel.
According to the invention, a decoupling of the voltage position and frequency position of the frequency converter output voltage or frequency converter output voltages is effected by means of the DC/DC converter or DC chopper. As a result of the thus possible low inverter switching frequency (fundamental clocking or block clocking) or of a unidirectional DC chopper operation, it is possible to use unipolar power switches (MOSFETs). Due to the possibility of connecting the circuit breakers in parallel, a power loss can be reduced to such an extent that cost-effective new concepts of structure and cooling become possible. By using unipolar power semiconductors connected in parallel, both in the DC/DC converter and in the inverter, output powers within a range of several kW can be provided for example by means of SMD power semiconductors without massive heat sinks.
However, apart from MOSFETs, bipolar power semiconductors such as IGBTs can also be used. Combinations of MOSFETs and IGBTs are also conceivable.
The frequency converter can have a multiphase rectifier for generating the input direct voltage.